Bis(aryloxyalkyl)terephthalates have been used to make adhesives, recording media, crystallization accelerators, and photosensitive layers. Recently, their use as gas-barrier additives for thermoplastic polyesters such as polyethylene terephthalate (PET) has been disclosed (see, e.g., U.S. Pat. Appl. Publ. Nos. 2010/0143546 and 2010/0143547). The additive functions as an antiplasticizer: it enhances modulus and tensile strength, thereby reducing creep when bottles are stacked. Additionally, it reduces the permeability of PET to carbon dioxide and oxygen, which improves the shelf life of carbonated beverages, particularly for smaller bottle sizes, and oxygen-sensitive drinks such as juice, tea products, or beer (see also U.S. Pat. Appl. Publ. No. 2006/0275568). As demonstrated in the '546 publication, adding just 3 wt. % of bis(2-phenoxyethyl)terephthalate increased barrier improvement factor by almost 20% and added about two weeks to shelf life. The process for making the bis(aryloxyalkyl)terephthalate is not discussed.
In a typical laboratory setting, bis(aryloxyalkyl)terephthalates are made by reacting terephthaloyl chloride with the corresponding aryloxyalkanol. For example, the reaction of terephthaloyl chloride with 2-phenoxyethanol in the presence of excess triethylamine and a solvent, followed by an organic workup to remove the ammonium salt, concentration, and recrystallization provides bis(2-phenoxyethyl)-terephthalate (see Scheme 1, compound 2 and Synthetic Examples 1 and 2 in U.S. Pat. Appl. Publ. No. 2009/0087764). Unfortunately, the lab procedure is impractical on a commercial scale because of the cost of terephthaloyl chloride and, among other issues, the need to recover a solvent and dispose of the ammonium salt.
In another laboratory approach (U.S. Pat. No. 3,557,167, Example 13), bis(2-phenoxyethyl)terephthalate is prepared by reacting diphenyl terephthalate with ethylene carbonate (1:3 molar ratio) in the presence of lithium chloride, followed by recrystallization from benzene. Limiting commercial use here are the need to synthesize diphenyl terephthalate (usually from terephthaloyl chloride), the relatively high cost of ethylene carbonate, and material losses when carbon dioxide is eliminated as a by-product.
Direct esterification of alcohols with terephthalic acid (TA) offers hope of a simpler purification by taking advantage of an acidic starting material and a neutral product. However, temperatures in excess of 250° C. (see, e.g., U.S. Pat. No. 4,737,569 or WO 82/00289) are normally required, and the potential ease-of-isolation advantages are undermined by the relatively poor solubility of TA compared with that of dimethyl terephthalate (DMT). Based on our own work, the approach may also impart unacceptably high color when the aryloxyalkanol is reacted with TA.
In one approach to making bis(aryloxyalkyl)terephthalates, the aryloxyalkanol is reacted with dimethyl terephthalate in the presence of a transesterification catalyst. Because the reaction is equilibrium controlled, it is difficult to convert substantially all of the DMT to a bis-ester, and the product can contain too much mono(aryloxyalkyl)terephthalate. An excess of the aryloxyalkanol can be used to shift the equilibrium toward completion. Unfortunately, side reactions can complicate this process, resulting in discoloration of the product. For a clear plastic bottle application, however, low color is important.
Additional problems result from the use of certain traditional condensation catalysts for the esterification. In particular, if residues from catalysts containing cobalt, manganese, cadmium, magnesium, and other metals are not avoided or thoroughly removed from the bis(aryloxyalkyl)terephthalates, they can cause an undesirable reduction in the molecular weight and intrinsic viscosity of the PET plastic into which the antiplasticizer is formulated, resulting in inferior blow-molded bottles (see U.S. Pat. Appl. Publ. No. 2006/0275568).
Some processes give low conversion to the desired bis-ester, while others generate the desired product, but in low yield or with too high an acid number, hydroxyl number, or moisture content. However, all of these considerations can be important when the bis(aryloxyalkyl)terephthalate is destined for making blow-molded PET bottles.
Yet another consideration is how easily the antiplasticizer can be processed uniformly with thermoplastics. Certain bis(aryloxyalkyl)terephthalates tend to be rather brittle, especially when produced at high purity and crystallinity. However, the antiplasticizer, when converted to pellets, prills, pastilles, flakes, granules or other articles, needs adequate crush strength so that it can be shipped or stored in gaylords, rail cars, tank trucks, or the like without disintegrating and forming dust. Moreover, when the antiplasticizer articles are combined with pellets of the thermoplastic polyester (e.g., PET), they should remain capable of shipping and storage without forming dust or separating from the dry-blended mixture.
In sum, bis(aryloxyalkyl)terephthalates are valuable antiplasticizers for thermoplastic polymers, especially PET, but a viable commercial synthesis is still needed. A suitable process would avoid pitfalls of a laboratory-scale process such as expensive starting materials, use of a solvent, and isolation/disposal of an ammonium salt. Preferably, the process would allow recycle and reuse of reactants and could be practiced using conventional esterification equipment and techniques. The process should avoid catalysts that reduce PET molecular weight during formulation into blow-molded articles. A valuable process would provide high yields of bis(aryloxyalkyl)terephthalates having the low color, low acid number, and low hydroxyl number needed to make the product acceptable for use as an antiplasticizer in the production of blow-molded thermoplastics. Ideally, the antiplasticizer could be formulated in a way that enables it to be stored and shipped without disintegrating, thereby ensuring an even distribution when it is combined and melt processed with thermoplastic polymers.